1. Technical Field
This invention relates to lithium ion batteries, and more particularly to a method of making electrodes (i.e., anodes and cathodes) therefor.
2. Description of the Related Art
Lithium ion batteries of the so-called xe2x80x9crocking chairxe2x80x9d type are known in the art and comprise a lithium-intercalateable anode, a lithium-intercalateable cathode and a lithium-ion-conductive electrolyte sandwiched therebetween, as seen generally by reference to U.S. Pat. No. 5,196,279 to Tarascon. One particular variant of such battery is the so-called xe2x80x9clithium polymerxe2x80x9d battery wherein (1) the electrodes (i.e. anode and cathode) contain lithium-intercalateable particles bound together in a porous polymer matrix, impregnated with electrolyte, and (2) a porous polymeric membrane/separator, impregnated with electrolyte, lies interjacent the electrodes.
It is known to fabricate lithium-polymer cells by sandwiching a thin dry film of the separator/membrane material between a thin dry film of anode material and a thin dry film of cathode material and forming a laminate thereof by bonding the several films together under heat and pressure. Current collecting grids may be pressed into the anode and cathode materials at the same time or in a separate operation. However, this approach involves many steps, which increase fabrication cost and complexity. Moreover, achieving consistent and enduring lamination has been an ongoing problem in the manufacture of lithium polymer batteries. Delamination of one or more layers may result in an inoperative battery.
Other approaches have been taken in the art. U.S. Pat. No. 5,296,318 to Gozdz et al. disclose a process for making a lithium polymer cell by a process wherein (1) a first electrode film is cast wet and dried on a first current collector defined by aluminum collector foil, (2) a separator/membrane film is cast wet and dried atop the first electrode film, (3) a second electrode film is cast wet and dried atop the separator/membrane, and (4) a second current collector applied to the second electrode film. However, the approach is not effective for mass production inasmuch as the process produces incomplete and/or unenduring contact between layers and components thereof. This is more particularly true for the above-mentioned lamination approach. The foregoing results in lower production efficiency, increased scrap rate (due to higher than acceptable resistances), and, accordingly, higher costs.
In copending application Ser. No. 09/862,388, filed May 21, 2001, assigned to the common assignee of the present invention, a process is disclosed for fabricating composite electrodes that involves the use of a carrier layer upon which an electrode is formed. Copending application Ser. No. 09/862,388 further discloses, in one embodiment, that the carrier layer is stripped off to separate the electrode therefrom. It would be desirable to avoid having to strip off the carrier layer.
There is therefore a need to provide an improved process for fabricating composite electrodes, including multilayer structures for use in lithium ion batteries or cells that minimizes or eliminates one or more of the problems as set forth above.
Manufacturing complexity, cost, and scrap rate can be reduced, production rates increased, and better contact between the grid and the electrode material achieved by a process according to the present invention.
The invention involves using a separator as a backing film in the coating process. Since the separator is used in cells and batteries, it can be retained, unlike the carrier disclosed in copending application Ser. No. 09/862,388, which is removed and discarded. This improvement allows attachment of the separator at an earlier stage in the overall process, and further, eliminates steps. In a preferred embodiment, the separator is a polymeric backing film. The separator performs its conventional function as well as a new function, namely, that of a carrier or backing film.